This invention relates to an amplifier having digital micro-processor control apparatus and, in particular, to a high frequency power amplifier that includes a micro-processor control system to accurately regulate the operating point of the various amplifying elements in the high frequency power amplifier.
The gain of an amplifier, especially a power amplifier will change at different frequencies, temperatures and operating levels, and between different units, unless the design compensates for these effects. It is a problem in the field of amplifiers to accurately and dynamically control the operation of the amplifying elements that comprise the amplifier.
In the case of amplifiers employed in communications systems, fast response times are required together with high output powers. Further, the input power and frequency can be different in successive bursts, so a new power level/attenuation must be set, for example in GSM applications, every 577 xcexcs. This means that the gain control must be fast, which limits the amount of processing that can be done in real time.
A problem typically encountered in amplifiers is that the amplifying elements, exhibit a fairly significant variation in their characteristics due to manufacturing tolerances. In addition, variations in operating temperature cause a shift in the operating point of these elements as does ageing of the amplifying elements. It is a typical procedure to fine tune the amplifier operation during the amplifier manufacturing process to compensate for the diversity of manufactured amplifying elements. Dynamic changes in operating environment can be compensated for by analogue feedback circuitry that is typically found in an amplifier. This analogue feedback circuitry can provide some rudimentary control over the quiescent point of the amplifying element, although these feedback schemes typically can not compensate for variation in the operating characteristics of the devices.
In the embodiment shown in FIG. 1, the power amplifier also employs a microprocessor to change the Quiescent Collector Current (Icq) to three cascaded class AB bipolar power transistors possibly every GSM burst (577 xcexcs) in the case of GSM applications, depending on the power amplifier output power (GSM base stations use up to 42 dB power control on each burst depending mainly on the mobile position.). The lcq is changed to make correction of the transistors"" self-heating effect possible. The power transistor also suffers from an internal self-heating effect; at high powers, the junction gets hotter, and the voltage applied to the base (Vbe) requirement is less to maintain the same lcq; this is normal functioning of the class AB transistor and the collector current (Ic) due to the dc bias goes up with increasing power (This can only be seen by measuring Ic immediately after the rf is removed.). However, a transistor has a thermal time constant and there is a transition period between different dissipated powers where the lcq is incorrect for maintaining near constant gain; this has to be compensated for with the bias.
A short time constant heating effect which is observed as rounding of the GSM burst edges (known as interburst ripple), and a long time constant heating effect which is observed as change in burst gain dependant on the power history (known as history effect). The bias shaping circuit corrects for the interburst ripple with a polarized short decay differentiator fed through a very short time constant integrator. The history effect is corrected by a long time constant integrator.
One common solution to the first problem is to use a number of trimmers which are adjusted during production test to compensate for some of these effects individually. Such trimmers may be used to supplement the performance of a variable voltage attenuator which would be under the control of a micro-controller. The use of trimmers increases the set-up time, which adds to the cost of the product. If a micro-controller and variable voltage attenuator are also used then the component costs together with the costs of setting trimmers and other associated equipment would be high.
It is possible, in theory to correct for static gain by adjusting the icq to compensate for the gain expansion curves; however, this does not correct for the dynamic signal and the history effect and interburst ripple effect was observed which made the gain out of specification. Two integrators with the Icq never going beyond lcqmin and lcqmax can also be employed, but this does not fully achieve dynamic gain specification. Additionally, a polarized differentiator was required to fully correct gain for interburst ripple.
U.S. Pat. No. 4,924,191 provides an amplifier arrangement having digital bias control apparatus that uses a processor to provide precise, dynamic control over the operating point of a plurality of amplifying elements in an amplifier. This processor controls each amplifying element to optimise the operating point of each individual amplifying element as a function of the amplifying element characteristics, the operating environment and the applied input signal. If the measured values of bias signal and output signal do not match predetermined desired values as stored in the processor memory, the processor updates the predefined bias value that is stored in memory to therefore shift the nominal operating point of this amplifying element to compensate for dynamic changes in the operating environment or the operating characteristics inherent in this particular device. This disclosure sets bias employing dynamic feedback but does not, however, provide a dynamic pulse shaping circuit.
The present invention seeks to provide an improved apparatus and method for controlling a power amplifier. The present invention also seeks to provide an efficient and inexpensive method of dynamically maintaining a constant gain in a power amplifier whilst the input power, input frequency and temperature are varying. The present invention further seeks to provide an amplifier circuit which compensates for variations in gain due to self-heating effect in a dynamic fashion. The present invention further seeks to provide an amplifier circuit which compensates for variations in gain with changing output power.
In accordance with a first aspect of the invention, there is provided an amplifier circuit comprising an amplifier chain, a micro-controller, a variable voltage attenuator (VVA), a digital to analogue converter and an EEPROM, wherein the EEPROM provides a lookup table which is read by the micro-controller, which is operable to the digital to analogue converter to set the control voltage to the variable voltage attenuator. The control voltage is updated continuallyxe2x80x94every 577 xcexcs, for example, in a GSM application. The lookup table can be derived by measuring a few points at room temperature for each power amplifier and then using previously measured typical data to calculate a full lookup table to compensate each power amplifier. A routine is provided whereby to compensate the amplifier gain via the variable voltage controller for the following variables: Unit to unit gain variation; Unit to unit frequency response; Unit to unit gain variation as a function of back off; and Unit to unit gain variation as a function of temperature. The routine takes data measured from individual amplifiers during production set up along with known xe2x80x98typicalxe2x80x99 responses and derives a set of EEPROM coefficients which are provided to amplifier controllers. The micro controller uses the EEPROM as a simple look up table to derive variable voltage controller settings.
The routine comprises of 3 parts: that of data gathering during unit test; that of calculating operating coefficients; and that of running the amplifier using the coefficients. The actual calculations involved in deriving the coefficients are performed in circuitry, for example in a computer outside the amplifier, and can be made as complicated as required, since there is no real time constraint on this operation.
In accordance with a still further aspect of the invention, there is provided a power amplifier which also employs a microprocessor to change the Quiescent Collector Current (Icq) to some class AB bipolar power transistors in the amplifier. The lcq is changed to make correction of the transistors"" self-heating effect possible. This provides a good efficiency which is obtainable at high powers and nearly constant gain achievable from the transistor at different power levels.
In accordance with a still further aspect of the invention, there is provided a bias shaping circuit, which uses a polarized differentiator with integrator combined with another integrator can then be applied to the transistor bias. Here, the micro-controller is employed to set Icq, as a linear function of power dissipated. A waveform is thus synthesised, which waveform is shaped with differentiators and integrators.
In sliding the bias, the output from digital to analogue converter is varied during each burst. (This voltage controls Quiescent Collector current, Icq). This allows near constant gain with output power and enables a low Icq at high powers which improves efficiency.
The invention thereby provides an efficient and inexpensive method of dynamically maintaining a constant gain in an amplifier whilst the input power, input frequency and temperature are varying. A relatively few points are measured on each amplifier to provide this compensation from amplifier to amplifier without the need to adjust trimmers on each unit, thereby reducing production test time. This present invention offers a simple, inexpensive and easy to set up automatic test facility correction method for the production of signal amplification circuitry. The present invention also avails itself to the simplifying of automation during the production of amplifiers.